In general, a gasoline engine has a principle that air and fuel are uniformly mixed before initiation of combustion, and then are ignited through an ignition plug to be combusted, and a diesel engine has a principle that air is only taken in and is compressed with a high compression ratio, and then fuel is injected to the compressed air at a high pressure, thereby reaching auto-ignition combustion.
Especially, a general diesel engine mainly adopts a method of combusting fuel by mixing the fuel and air well by making fuel injected from an injector swirl in a combustion chamber (bowl) provided at a piston.
The combustion chamber of the diesel engine needs to be designed so as to maintain low smoke and a state where smoke is not worsened even at a delay of an injection time. That is, the combustion chamber of the diesel engine needs to facilitate formation of air-fuel mixture by making injected fuel collide with a wall surface of the combustion chamber, maintain a swirl flow inside the combustion chamber, suppress a flow of fuel vapor to a squish region formed between a piston and a cylinder head, and stimulate an air flow.
The diesel engine exhibits engine performance by injecting fuel into the combustion chamber at an appropriate time and mixing the injected fuel and intake air to facilitate combustion. In this case, the injection of the fuel is started at an end stage of a compression stroke, the fuel is mixed with air by the swirl flow inside the combustion chamber from the end stage of the compression stroke to an initial stage of an explosion stroke to be combusted, and then non-combusted smoke is mixed with air at a top portion of the piston by the squish flow to be further combusted, thereby suppressing a smoke generation.
Accordingly, in order to improve engine performance and solve exhaust gas, the diesel engine needs to make the combustion chamber have an optimized shape so as to realize optimal mixing of air and fuel inside the combustion chamber.
FIG. 1 is a cross-sectional view of a combustion chamber of a diesel engine in the related art, and the diesel engine is provided with a combustion chamber 2 shaped like a bowl enabling fuel injected from an injector 3 to swirl and squish at an upper portion of a piston 1.
The piston 1 vertically moves while sliding against an inner circumferential surface of a cylinder block to compress or expand air inside the combustion chamber 2, and simultaneously the fuel injected from the injector 3 is mixed with the air.
Accordingly, when high pressure fuel is injected from the injector 3 in a state where the intake air through an intake port is compressed by an ascending movement of the piston 1, the fuel injected from the injector 3 is mixed with the air forming turbulence while creating a swirl in the combustion chamber provided on an upper surface of the piston 1.
However, the combustion chamber of the diesel engine has a problem in that swirl intensity is decreased under a condition in which the combustion chamber is operated in a low-speed operation region, such that engine performance deteriorates and particulate matters (PM) and nitrogen oxide (NOx) are increased as well in an aspect of exhaust gas.
In order to solve the problem, an effort, such as a height of the injector (fuel injection nozzle) may be adjusted, or a shape of the combustion chamber is formed to have a radial shape as can be seen in a piston structure of a diesel engine of Korean Patent Application Laid-Open No. 10-2005-0036098, has been made.
However, the combustion chamber of the diesel engine has a problem in that mixing of the fuel and the air is not smooth, causing flow stagnation, and combustion concentration is generated due to the flow stagnation, such that a temperature is partially increased, thereby increasing nitrogen oxide.